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The present invention relates in general to a musical instrument transducer. More particularly, it relates to a piezoelectric transducer used with a stringed musical instrument such as a guitar.
The prior art shows a variety of electromechanical transducers employed with musical instruments, particularly guitars. Many of these transducers are not completely effective in faithfully converting mechanical movements or vibrations into electrical output signals which precisely correspond to the character of the input vibrations. This lack of fidelity is primarily due to the nature of the mechanical coupling between the driving vibrating member (i.e. a string) and the piezoelectric material of the transducer. Some of the prior art structures, such as those shown in U.S. Pat. Nos. 4,491,051 and 4,975,616, are also quite complex in construction and become quite expensive to fabricate. Furthermore, a transducer using a piezoelectric material requires a conductive layer, a ground layer, and some form of shielding to prevent electrical interference. These multiple layers not only increase the complexity of the transducer, but interfere with the ability to attach leads to the transducer as it is made smaller to operate in a musical instrument.
Differently shaped transducers have been produced for musical instruments. Generally, transducers for stringed instruments have a flat, elongated shape. The piezoelectric layer for such transducers can also be elongated, or can be individual crystals between electrodes. Alternatively, one prior art transducer was coaxially arranged, with a center electrode, surrounding piezoelectric layer, and outer electrode, as illustrated in U.S. Pat. No. 4,378,721.
Each shape offers unique difficulties in construction and varying degrees of quality in operation and performance. For good performance, the piezoelectric layer needs to respond to small string movements at a variety of frequencies. With a thicker layer of piezoelectric material, the material needs to be more flexible; if made too thick, the piezoelectric layer may be too brittle for the intended use, and may not provide satisfactory response characteristics across of range of input stimuli including the smallest string movements. To achieve sufficient resilience in a coaxial arrangement, U.S. Pat. No. 4,378,721 discloses a material formed from a rubber material mixed with a powdered piezoelectric ceramic and a vulcanizing or cross-linking agent. Piezoelectric ceramic is typically brittle and inflexible. This reference relies upon a rubber matrix to bind together the powdered ceramic material. The use of a rubber material results in a significantly thicker piezoelectric material layer, which is inconsistently responsive across a variety of input frequencies; the rubber matrix tends to damp input stimuli, resulting in degraded response. A thicker piezoelectric layer, even if comprised of rubber, becomes more difficult to physically accommodate, to bend or to otherwise manipulate. Over time, it has been found that the composite piezoelectric layer such as described in this reference tends to deform in response to compression such as is typical in a stringed instrument application.
A further disadvantage of the coaxial transducer as described in U.S. Pat. No. 4,378,721 relates to its formation through a casting or molding process, such that the length of the resulting transducer is dependent on the size of the molds available. Other manufacturing processes are not suitable for the composite piezoelectric material due to a low degree of cohesiveness.
Additionally, the polarization of the piezoelectric material of this reference must be performed after completion of the casting procedure. Two opposing, plate-like electrodes, on either side of the transducer, are used to initialize the magnetic domains of the piezoelectric material, thereby complicating and extending the manufacturing process of such a transducer. Therefore, a need exists for an accurate, responsive transducer with a thin, relatively stiff piezoelectric layer which can be economically formed into a coaxial arrangement.
The deficiencies of the prior art are substantially overcome by the transducer according to the present invention, which includes a coaxial structure having a central conductor, a piezoelectric polymer layer, and an outer conductor. The central conductor may be formed of a wire bundle or a solid wire. A piezoelectric cylinder of either a piezoelectric copolymer or a monopolymer is formed about the central conductor. The piezoelectric material may be substantially thinner than that of the prior art, thus providing significantly improved response characteristics for the output signal, while providing a desired degree of flexibility and resistance to deformation over time.
The outer conductor can be formed as a braided sheath or simply as a conductive paint on the outside of the piezoelectric material. Other embodiments include the use of conductive foil, conductive shrink tubing, or any other flexible, conductive material which has a minimal impact on the flexibility of the overall transducer and on the response characteristics of the piezoelectric material. An additional mechanically shielding layer may also be provided, though this layer must not significantly interfere with the responsiveness of the transducer. Leads are attached to the central and outer conductors in order to complete the transducer. The coaxial transducer may be provided with a length sufficient to fit within the saddle of a guitar, underneath the strings. Other embodiments may be configured for use with other stringed musical instruments.
An alternative embodiment of the presently disclosed transducer employs a piezoelectric polymer material provided in the form of a film tape. The tape may be helically wrapped or woven about a central conductive core. An outer conductive layer, provided by a conductive foil, paint or tubing layer, is disposed about the piezoelectric film tape. Further still, an electromechanical film such as an electret film containing an electric charge may be employed in the place of the piezoelectric layer.